1. Field of the Invention
The present invention relates generally to receivers and particularly to receivers having capability to detect dynamic multi-path effects.
2. Description of the Prior Art
In a communication system, information is sent from a transmitter to a receiver over a channel, which introduces various impairments. One type of impairment that is commonly encountered in wireless communications is called multi-path. It is so-called because multiple copies, or “echoes”, of the transmitted signal arrive at the receiver, taking multiple paths through the wireless medium. In analog television systems, such as NTSC in North America and PAL in Europe, multi-path impairments cause multiple images or “ghosts” in addition to the primary image. In wireless digital communication systems, including digital terrestrial television systems such as those defined by: 1) The known standard A/53 adopted by the Advanced Television Systems Committee (ATSC); 2) The known standard EN 300 744 adopted by the European Telecommunications Standards Institute (ETSI); or 3) The known standard for Digital Television Terrestrial Multimedia Broadcast (DTMB) adopted in China, it is generally necessary to mitigate multi-path in the receiver in order to decode the in-coming signal without errors.
As is well known in the art, multi-path impairments can be categorized as either “flat fading” or “frequency-selective fading.” In general, flat fading refers to multi-path impairments that cause approximately equal attenuation at all frequencies in the signal band. This type of fading can be compensated by a simple multi-path mitigation device, such as an Automatic Gain Control (AGC.) Frequency-selective fading, on the other hand, attenuates some frequencies more than others. This necessitates the use of a more complex multi-path mitigation device such as an equalizer. Generally the receiver does not know in advance the exact attenuation of each frequency, so it must use an adaptive multi-path mitigation device such as an adaptive equalizer, which must be adapted so as to allow error-free recovery of data. Initial adaptation occurs at the beginning of the signal reception. Such adaptation may include programming initial coefficients, updating coefficients based on either blind or deterministic criteria, or other adaptive approaches. After initial equalizer adaptation is complete, the adaptation can be slowed or stopped, assuming the multi-path is static.
However, if the multi-path is dynamic, such as is the case when the receiver is moving, then the relative attenuation of different signal frequencies changes over time and needs to be tracked by a dynamic multi-path mitigation device, such as a continuously training adaptive equalizer.
As is well known in the art, one challenge in designing a dynamic multi-path mitigation device, such as a continuously training adaptive equalizer, is choosing the appropriate adaptation speed for the equalizer. However, typically, faster adaptation results in increased sensitivity to noise, thus, there is a tradeoff between noise performance and tracking speed. Various approaches have been considered in the art for automatically adjusting the adaptation speed of an adaptive equalizer. Many of these approaches rely on the error level after equalization, based for example on constellation distance measurements or bit error corrections performed by a forward error correction (FEC) module like a Reed-Solomon decoder. For example, in one prior art technique, successive values of mean square error (MSE) are used to adjust the adaptation speed of an equalizer. Other prior art techniques adjust the adaptation speed of the equalizer in a direction to reduce packet error rate and do so within the Reed-Solomon decoder.
The problem with relying on error level for adjusting the adaptation speed is that it is difficult to ascertain the source of an increase in error level. That is, it is not known whether or not the source of the increased error level is due to dynamic multi-path, requiring a faster adaptation speed or due to random noise, requiring a slower adaptation speed. Problems associated with the inability to correctly identify the source of the experienced increase in error level obviously lead to various issues. For example, it may lead to erroneous adjustments and more importantly to failure to mitigate dynamic multi-path.
Some prior art techniques do not address the problem of detecting the source of the increased error level and others rely on a trial and error type approach. Yet another approach is to adjust the adaptation speed using the gradient of the equalizer coefficients. For example, in one prior art method, the equalizer step sizes are adjusted based on the gradient of the coefficients. However, the coefficient gradient is sensitive to random noise, thus, this approach can cause the adaptation speed to be unnecessarily and undesirably increased.
The use of multiple filters with different time constants for detection purposes is known in the art of signal processing but is not known to have been applied to dynamic multi-path detection. In a known prior art technique, in detecting voice activity, a fast filter and a slow filter are used to distinguish between real voice signals and noise signals and the filters are used to measure the power of the entire signal but they are not used to measure the signal metric in a frequency band less than the input signal bandwidth. The problem with using this approach for dynamic multi-path detection is that in many cases, dynamic multi-path causes significant change in the signal metric in a particular frequency band but the overall signal power remains the same. Therefore, the change in the signal metric would remain undetected and dynamic multi-path would not be mitigated.
With the shutoff of analog terrestrial television looming, the performance demands on digital television receivers are increasing. A particular area of interest is performance in dynamic multi-path environments where the digital television receiver is mobile. Additionally, there is a requirement that improved dynamic multi-path performance not occur at the expense of increased sensitivity to additive noise in static channels.
In light of the foregoing, there is a need for a method and apparatus for the detection of dynamic multi-path for use in configuring the adaptation speed of a dynamic multi-path mitigation device, such as a continuously training adaptive equalizer.